A Schottky barrier diode (also known as a surface-barrier diode and as a metal-semiconductor diode) comprises a semiconductor substrate (such as silicon or gallium arsenide) which has a metal contact (such as gold, platinum, palladium or silver) formed on a localized area of one substrate face. The diode formation has been previously achieved by evaporation techniques or by ion implantation through the surface of the semiconductor. An electrostatic barrier characteristically exists at the metal-semiconductor interface which causes the interface to have rectifying properties.
In the case of an n-type semiconductor, for example, when a forward bias (that is, a positive voltage V.sub.F) is applied across the interface, current flows freely and majority carriers are injected into the metal side of the interface (or junction) where they remain majority carriers at some energy greater than the Fermi energy of the metal. When a reverse voltage V.sub.R is applied across the same interface, the current flow is blocked. Switching from the V.sub.F "on" state to the V.sub.R "off" state can occur in an extremely short time (in the order of picoseconds). No stored minority-carrier charge exits. The reverse dc current-voltage characteristics of such a diode are very similar to those of conventional pn junction diodes.
For present definitional purposes, FIG. 1 includes an energy band diagram for a Schottky barrier junction 10 under a thermal equilibrium bias state. Electrons flow from the semiconductor 11 into the metal of the contact 12 until the energy of the electrons in the contact 12 and the semiconductor 11 are equal. Thus, the Fermi levels of the metal and semiconductor E.sub.F.sup.m and E.sub.F.sup.S, respectively, are equal and the resulting level is shown in FIG. 1 as E.sub.F. The electrons which flow out of the semiconductor 11 leave behind immobile, positively charged dopant ions within the (n-type) semiconductor 11 that create an electric field within the semiconductor 11. E.sub.C and E.sub.V are the conduction and the valence band edges, respectively, E.sub.g is the band gap, and 14 is the valence band in metal contact 12. The bending of the energy bands at the interface 13 between metal contact 12 and semiconductor 11 occurs in such a manner as to retard the flow of majority carriers into the metal contact 12. The work function of the metal contact 12 is indicated by .phi..sub.m. The electron affinity X.sub.s of the semiconductor 11 is also indicated.
The Schottky energy barrier height .phi..sub.Bn that is present at the interface 13 is thus defined as the energy difference between the Fermi level in the metal contact 12 and the conduction band E.sub.c minimum in the semiconductor 11 at the interface 13. It has previously been found that .phi..sub.Bn is essentially independent of the semiconductor doping level.
Schottky described the relationship between the barrier height .phi..sub.Bn, the work function .phi..sub.m of the contact metal 12 and the electron affinity X.sub.s of the semiconductor 11 by the relationship: EQU .phi..sub.Bn =.phi..sub.m -X.sub.s ( 1)
which is the so-called "Schottky model". This model predicts that the Schottky barrier height .phi..sub.Bn is proportional to the work function .phi..sub.m of the contact metal 12 so that, by choosing different metals, contact behavior can be varied so as to range from ohmic to rectifying characteristics.
Unfortunately, this model does not hold for metal contacts on gallium arsenide (GaAs)-type substrates. For most metal/GaAs interfaces, .phi..sub.Bn is independent of the metal; that is, Fermi level pinning is observed. Also, most metals are not in thermodynamic equilibrium with GaAs. An elemental metal is not useful as a contact for a GaAs substrate if long-term stability is to be maintained.
For quantification purposes, three methods are known for measuring the barrier height .phi..sub.Bn of a metal-semiconductor contact: (1) the current voltage method (I-V), (2) the capacitance-voltage method (C-V), and (3) the internal photoemission methods. The respective values of .phi..sub.Bn obtained using each measuring method often do not agree with one another.
In the I-V method, the thermionic-emission mechanism is assumed. The relationship between the current density J and the applied forward voltage V is given by the equation: EQU J=A**T.sup.2 exp[-q(.phi..sub.Bn -.DELTA..phi..sub.Bn)/.kappa.T][exp(qV/n.kappa.T)-1] (1.1)
in which A** is the effective Richardson constant (8.64 A/cm.sup.2 -K.sup.2 for n-GaAs), .kappa. is the Boltzmann constant, T is the temperature, .phi..sub.Bn is the Schottky barrier lowering [x=(qE/4.pi..epsilon..sub.s).sup.1/2 ], where E is the maximum electric field at the metal-semiconductor interface and .epsilon..sub.s is the permittivity of the semiconductor) and n is the ideality factor, which is an indication of the deviation of the contact from ideal thermionic-emission behavior. The ideality factor, n, has to be close to 1 to assure the applicability of the above equation.
If V&gt;3.kappa.T/q, Eq. (1.1) can be simplified as follows: ##EQU1## In a plot of in J vs. V, the slope yields the ideality factor n via the relation EQU n=q/.kappa.T*[.sup..differential. V.sup..differential. (1n J)](1.3)
and the intercept on the 1n J axis yields the saturation current J.sub.s which may be used to determine .phi..sub.Bn : EQU .phi..sub.Bn =.kappa.T/q[1n(A**T.sup.2 /J.sub.s)]+.DELTA..phi..sub.Bn( 1.4)
The .phi..sub.Bn and n values are not sensitive to errors in A** because A** is inside the logarithmic term.
In the C-V method, use is made of the fact that when a small alternating current (dc) voltage is superimposed upon a direct current (dc) bias, charges of one sign are induced on the metal surface and charges of the opposite sign are induced in the semiconductor. Based on the depletion theory, the relationship between the capacitance at the metal-semiconductor interface and the Schottky barrier height can be derived as: EQU 1/C.sup.2 =2(.phi..sub.Bn -V-.kappa.T/q)/q.epsilon..sub.s N.sub.D( 1.5)
where N.sub.D is dopant concentration in the semiconductor, s is the permittivity of the semiconductor, C is the depletion-layer capacitance and V is the applied reverse voltage. By plotting 1/C.sup.2 vs. V, the dopant concentration N.sub.D may be obtained from the slope of the straight line and the barrier height .phi..sub.Bn may be calculated as the follows: EQU .phi..sub.Bn =V.sub.i +V.sub.n +.kappa.T/q (1.6)
where V.sub.n is the depth of the Fermi level below the conduction band and Vi is the intercept of the voltage axis. This method is valid only when the dopant concentration in the semiconductor is uniform.
The existence of traps at the metal-semiconductor interface will affect the applicability of Equation (1.6). It has been shown that the occupation of the traps is frequency-dependent. To avoid errors caused by the traps, the test frequency must be high so that the charges of the traps cannot follow the frequency.
In the internal photoemission methods, the photoresponse measurement is an accurate and direct method of determining the barrier height .phi..sub.Bn. When monochromatic light is incident upon a semiconductor surface, a photocurrent may be generated. The photocurrent per absorbed photon, R, as a function of the photon energy, h.nu., is given by the Fowler theory: ##EQU2## where h .nu..sub.o is the barrier height (q.phi..sub.Bn). When the square root of R is plotted as a function of photon energy, a straight line should be obtained, and the extrapolated value on the energy axis should give directly the barrier height .phi..sub.Bn.
For present purposes, the I-V and the C-V methods are presently preferred for measuring Schottky barrier height .phi..sub.Bn.
There is a great need in the art relating to Schottky diodes for individual, stable Schottky diodes which utilize n-GaAs-type substrates and which have a predetermined or tunable barrier height .phi..sub.Bn. A method for making such Schottky diodes is also needed. The ability to produce a Schottky diode with a predetermined barrier height .phi..sub.Bn would greatly enhance the ability of the semiconductor art to utilize such diodes particularly in the field of integrated circuit devices.
Similarly, there is a great need in the art of Schottky diodes for individual Schottky diodes having a wide range of predetermined and controlled barrier heights.
So far as now known, no one has previously described or suggested either a method for making a Schottky diode with a particular desired (or predictable) barrier height .phi..sub.Bn or a product Schottky diode with such a definite (or predictable) barrier height that is made by such a method.
The present invention is directed to and satisfies these needs.